Advances in various technologies have enabled a proliferation of portable devices for wireless communications. Examples of portable wireless communications devices (WCDs) include portable devices such as cellular telephones, wireless modems, pagers, personal e-mail devices (such as a Blackberry™), and personal digital assistants (PDAs). Such a device may be configured to provide voice and/or data services via communication with a network, such as a network for cellular telephony and/or packet data services or a wireless local-area network (WLAN) or personal-area network (WPAN), according to technologies and standards such as one or more of the following: code-division multiple access (CDMA), orthogonal frequency-division modulation (OFDM), a third-generation (3G) system for cellular communications, cdma2000, wideband CDMA (WCDMA), Global System for Mobile Communications (GSM), IS-95, current and future IEEE 802.XX standards, Bluetooth™, and ultra-wideband (UWB).
Some portable WCDs are configured to be self-contained and carry their own power source. Others are configured to provide wireless connectivity to and/or to obtain power from another portable device, such as a cellular telephone or other wireless modem configured to connect to a laptop or other portable computer via a PCMCIA slot or USB port. The description herein assumes that the portable power source is a battery (typically a rechargeable battery), but another power source such as supercapacitor and/or a fuel cell may also be used, possibly augmented by one or more solar cells.
It is desirable to minimize power consumption of a WCD so that the time between battery recharges may be maximized. It may also be desirable for the WCD to consume some power, even when the user is not actively using the device, such that the network may alert the WCD to become fully active when it receives notification of an incoming call directed to the device. It is desirable to minimize the amount of power that the WCD needs to consume in order to maintain this degree of connection to the network.
To reduce the power consumed by a WCD when the device is not actively engaged in communication with another terminal, a wireless network may be configured to page the WCD during predefined discrete intervals using a slotted paging channel. In a WCDMA system or Universal Mobile Telephone System (UMTS), for example, these discrete intervals occur during “paging occasions.” A WCDMA network is typically configured to transmit a paging indication channel (PICH) that contains 10-millisecond frames, each frame having 288 bits. The identification number of the WCD (also called “user equipment” or UE in WCDMA) determines which bits of certain PICH frames correspond to, and should be monitored by, that WCD. This identification number, also called an International Mobile Station Identifier (IMSI), is encoded onto the WCD's Subscriber Identity Module (SIM) card. The state of the PICH bits corresponding to the WCD indicate whether the device is being paged (for example, bits encoded as low values (zeroes) may indicate no page, while bits encoded as high values (ones) may indicate that the WCD is being paged). In some systems, the information received during the paging interval allows the WCD to respond to the page, while in other systems, the information received during the paging interval may indicate simply that such information is now available (e.g., on another channel). Other 3G systems use similar paging schemes (e.g., the quick paging channel (QPCH) of a cdma2000 network).
One option would be to keep the WCD awake all the time, and to demodulate all bits of the PICH. However, such an option would severely limit battery life. Thus it is desirable to cycle the WCD between a sleep period, during which various circuitry is powered down to conserve power, and an active period, during which the assigned paging interval is monitored (e.g., the assigned paging indicator bits are received and demodulated). Such a cycle is called a discontinuous reception (DRX) cycle.
One hardware block that is usually powered down during the sleep period is a master oscillator which supplies a source signal (typically of high frequency) for various other clocks in the WCD. The master oscillator may be implemented as a temperature-compensated crystal oscillator (TCXO). Other blocks that are usually powered down during the sleep period include RF circuitry and clocks for driving digital logic (for example, CMOS logic consumes very little power when it is not clocked).
A WCD operating according to a DRX cycle usually has a sleep timer configured to time the sleep period. The sleep timer typically includes a low-cost crystal oscillator having a much lower power consumption, frequency, and accuracy than the master oscillator. It may be expected that upon activation, timing errors will arise due to inaccuracy of the sleep timer. Timing errors may be accentuated in a case where the WCD executes a searching and/or synchronization algorithm at a suboptimal rate (for example, to conserve power).
A WCD may also incur frequency errors, which may arise in a variety of ways. For example, the master oscillator of a WCD is also imperfect, and although its frequency is typically calibrated by the manufacturer, the frequency may vary due to temperature and/or aging. Additionally, relative movement of the WCD with respect to a fixed portion of the network may result in an apparent frequency shift due to Doppler effects. Frequency errors may also lead to timing errors that complicate the task of maintaining synchronization with the network.
For proper operation of the WCD, it may be desirable to correct or compensate for these errors. Such correction or compensation takes time to execute, which time may be measured by processor cycles. The time occupied by these operations imposes an additional period during which the WCD stays awake and consumes power.